Low-Yield Earth-Penetrating Nuclear Weapons

By Robert W. Nelson

Fig. 1Diagrams like this one give the false impression that a low-yield earth penetrating nuclear weapon would "limit collateral damage" and therefore be relatively safe to use. In fact, because of the large amount of radioactive dirt thrown out in the explosion, the hypothetical 5-kiloton weapon discussed in the accompanying article would produce a large area of lethal fallout. (Philadelphia Inquirer/ Cynthia Greer, 16 October 2000.)

Despite the global
sense of relief and hope that the nuclear arms race ended with the Cold War, an
increasingly vocal group of politicians, military officials and leaders of
America's nuclear weapon laboratories are urging the US to develop a new
generation of precision low-yield nuclear weapons. Rather than deterring
warfare with another nuclear power, however, they suggest these weapons could
be used in conventional conflicts with third-world nations.

Critics argue that
adding low-yield warheads to the world's nuclear inventory simply makes their
eventual use more likely. In fact, a 1994 law currently prohibits the nuclear
laboratories from undertaking research and development that could lead to a
precision nuclear weapon of less than 5 kilotons (KT), because "low-yield
nuclear weapons blur the distinction between nuclear and conventional war."

Last year, Senate
Republicans John Warner (R-VA) and Wayne Allard (R-CO) buried a small provision
in the 2001 Defense Authorization Bill that would have overturned these earlier
restrictions. Although the language in the final Act was watered down, the Energy
and Defense Departments are still required to undertake a study of low-yield
nuclear weapons that could penetrate deep into the earth before detonating so
as to "threaten hard and deeply buried targets." Legislation for
long-term research and actual development of low-yield nuclear weapons will
almost certainly be proposed again in the current session of Congress.

Senators Warner and
Allard imagine these nuclear weapons could be used in small-scale conventional
conflicts against rogue dictators, while leaving most of the civilian
population untouched. As one anonymous former Pentagon official put it to the Washington
Post last spring,

"What's needed
now is something that can threaten a bunker tunneled under 300 meters of
granite without killing the surrounding civilian population."

Statements like these
promote the illusion that nuclear weapons could be used in ways which minimize
their "collateral damage," making them acceptable tools to be used
like conventional weapons.

As described in
detail below, however, the use of any nuclear weapon capable of destroying a
buried target that is otherwise immune to conventional attack will necessarily
produce enormous numbers of civilian casualties. No earth-burrowing missile can
penetrate deep enough into the earth to contain an explosion with a nuclear
yield even as small as 1 percent of the 15 kiloton Hiroshima weapon. The
explosion simply blows out a massive crater of radioactive dirt, which rains
down on the local region with an especially intense and deadly fallout.

Moreover, as Congress
understood in 1994, by seeking to produce usable low-yield nuclear weapons, we
risk blurring the now sharp line separating nuclear and conventional warfare,
and provide legitimacy for other nations to similarly consider using nuclear
weapons in regional wars.

Conventional Earth-Penetrating Weapons

Fig. 2The Pentagon has a growing collection of high precision conventional weapons capable of defeating hardened targets. In this sled-driven test, the GBU-28 laser guided bomb with its improved BLU-113 warhead penetrates several meters of reinforced concrete.

Fig. 3A B2 bomber releases an unarmed B61-11 earth-penetrating bomb during tests in
Alaska. Despite falling from an altitude of 40,000 feet, this bomb burrowed only approximately 20 feet into the soil. Any nuclear blast at this shallow depth would not be contained, and would produce intense local fallout.

The Pentagon already
has a number of conventional weapons capable of destroying hardened targets
buried within approximately 50 feet of the surface. The most well-known of
these is the GBU-28 developed and deployed in the final weeks of the air
campaign in the Gulf War. The Air Force was initially unable to destroy a
well-protected bunker north of Baghdad after repeated direct hits. The 4000 lb
GBU-28 was created from a very heavy surplus Army eight-inch gun tube filled
with conventional explosive and a modified laser guidance kit. It destroyed the
bunker, which was protected by more than 30 feet of earth, concrete and
hardened steel.

The precision,
penetrating capability, and explosive power of these conventional weapons has
improved dramatically over the last decade, and these trends will certainly
continue. Indeed, the GBU-37 guided bomb, a successor to the GBU-28, is already
thought to be capable of disabling a silo based ICBM  a target formerly
thought vulnerable only to nuclear attack. In the near future, the United
States will deploy new classes of hard target penetrators which can land within
one to two meters of their targets.

The B61-11 Nuclear Bomb

However, mini-nuke
advocates  mostly coming from the nuclear weapons labs  argue that low-yield
nuclear weapons should be designed to destroy even deeper targets.

The US introduced an
earth-penetrating nuclear weapon in 1997, the B61-11, by putting the nuclear
explosive from an earlier bomb design into a hardened steel casing with a new
nose cone to provide ground penetration capability. The deployment was
controversial because of official US policy not to develop new nuclear weapons.
The DOE and the weapons labs have consistently argued, however, that the B61-11
is merely a "modification" of an older delivery system, because it
used an existing "physics package."

The earth-penetrating
capability of the B61-11 is fairly limited, however. Tests show it penetrates
only 20 feet or so into dry earth when dropped from an altitude of 40,000 feet.
Even so, by burying itself into the ground before detonation, a much higher
proportion of the explosion energy is transferred to ground shock compared to a
surface bursts. Any attempt to use it in an urban environment, however, would
result in massive civilian casualties. Even at the low end of its 0.3-300
kiloton yield range, the nuclear blast will simply blow out a huge crater of
radioactive material, creating a lethal gamma-radiation field over a large
area.

Containment

Just how deep must an
underground nuclear explosion be buried in order for the blast and fallout to
be contained?

The US conducted a
series of underground nuclear explosions in the 1960s  the Plowshare tests 
to investigate the possible use of nuclear explosives for excavation purposes.
Those performed prior to the 1963 Atmospheric Test Ban Treaty, such as the
Sedan test shown in Figure 4, were buried at relatively shallow depths to
maximize the size of the crater produced.

Fig. 4The 100
KT Sedan nuclear explosion, one of the Plowshares excavation tests, was buried
at a depth of 635 feet. The main cloud and base surge are typical of
shallow-buried nuclear explosions. The cloud is highly contaminated with
radioactive dust particles and produces an intense local fallout.

In addition to the
immediate effects of blast, air shock, and thermal radiation, shallow nuclear
explosions produce especially intense local radioactive fallout. The fireball
breaks through the surface of the earth, carrying into the air large amounts of
dirt and debris. This material has been exposed to the intense neutron flux
from the nuclear detonation, which adds to the radioactivity from the fission
products. The cloud typically consists of a narrow column and a broad base
surge of air filled with radioactive dust which expands to a radius of over a
mile for a 5 kiloton explosion.1 In the Plowshare tests, roughly 50
percent of the total radioactivity produced in the explosion was distributed as
local fallout  the other half being confined to the highly-radioactive crater.

In order to be fully
contained, nuclear explosions at the Nevada Test Site must be buried at a depth
of 650 feet for a 5 kiloton explosive  1300 feet for a 100-kiloton explosive.2
Even then, there are many documented cases where carefully sealed shafts
ruptured and released radioactivity to the local environment.

Therefore, even if an
earth penetrating missile were somehow able to drill hundreds of feet into the
ground and then detonate, the explosion would likely shower the surrounding
region with highly radioactive dust and gas.

Long-Rod Penetration

Fig. 5Underground
nuclear tests must be buried at large depths and carefully sealed in order to
fully contain the explosion. Shallower bursts produce large craters and intense
local fallout. The situation shown here is for an explosion with a 1 KT yield
and the depths shown are in feet. Even a 0.1 KT burst must be buried at a depth
of approximately 230 feet to be fully contained. (Adapted from Terry Wallace,
with permission.)

It is straightforward
to show, however, that the maximum penetration depth is severely limited if the
missile casing is to remain intact. One can make reasonably accurate estimates
of the penetration depth based on the well-developed theory of "long-rod
penetration." The fundamental parameter R is the ratio of the
projectile ram pressure to the yield strength of the material.3 The
target material yields, and penetration occurs, when R is greater than
one. For a steel rod to penetrate concrete, the minimum velocities for
penetration is about one half a kilometer per second (1100 miles per hour). For
ductile materials, the kinetic energy lost from the penetrator can deform the
target and dig out a penetration crater.

Fundamentally,
however, the depth of penetration is limited by the yield strength of the
penetrator  in this case, the missile casing. Even for the strongest
materials, impact velocities greater than a few kilometers per second will
substantially deform and even melt the impactor.

An earth-penetrating
nuclear weapon must protect the warhead and its associated electronics while it
burrows into the ground. This severely limits the missile to impact velocities
of less than about three kilometers per second for missile cases made from the
very hardest steels. From the theory of "long-rod penetration," in
this limit the maximum possible depth D of penetration is proportional
to the length and density of the penetrator and inversely proportional to the
density of the target. The maximum depth of penetration depends only weakly on
the yield strength of the penetrator.4 For typical values for steel
and concrete, we expect an upper bound to the penetration depth to be roughly
10 times the missile length, or about 100 feet for a 10 foot missile. In actual
practice the impact velocity and penetration depth must be well below this to
ensure the missile and its contents are not severely damaged.

Given these
constraints, it is simply not possible for a kinetic energy weapon to penetrate
deeply enough into the earth to contain a nuclear explosion.

The Weapons Labs and the CTBT

The most vocal
proponents of new small-yield weapons come from the nation's nuclear weapons
laboratories, at Los Alamos and Livermore.

In a 1991 Strategic
Affairs article entitled "Countering the Threat of the Well-armed
Tyrant," Los Alamos weapons analysts Thomas Dowler and Joseph Howard II,
argued that the US has no proportionate response to a rogue dictator who uses
chemical or biological weapons against US troops. Our smallest nuclear weapons
 those with Hiroshima-size yieldswould be so devastating that no US president
could use them. We would be "self-deterred." To counter this dilemma,
they argued the US should develop "mininukes," with yields equivalent
to 0.01-1 KT: "... nuclear weapons with very low yields could provide an
effective response for countering the enemy in such a crisis, while not
violating the principle of proportionality."

"The US will
undoubtedly require a new nuclear weapon ... because it is realized that the
yields of the weapons left over from the Cold War are too high for addressing
the deterrence requirements of a multi polar, widely proliferated world.
Without rectifying that situation, we would end up being self-deterred."

A more cynical
interpretation of these statements is that the laboratory staff and leadership
simply feel threatened by the current restrictions on their activities, and
want to generate a new mission (and the associated funding) to keep them in
operation indefinitely. Indeed, beginning in 1990 with the collapse of the
Soviet Union and the end of the Cold War, there was serious discussion of
closing one of the bomb labs.

Moreover, President
Clinton ended US nuclear testing in 1993, and signed the Comprehensive Test Ban
Treaty (CTBT)  a permanent worldwide ban on nuclear testing  in 1996. Despite
the Senate's failure to ratify the CTBT in 1999, its proponents believe the
treaty will eventually come into force. The major nuclear powers continue to
abide by the world moratorium on nuclear testing, and even India and Pakistan
appear to have joined the moratorium after their May 1998 nuclear tests.

The nuclear weapons
labs are particularly threatened by the CTBT, since it will probably limit them
to maintaining the stockpile of weapons already in our arsenal. Keeping young
scientists interested in the weapons program is especially difficult when their
main job is the relatively mundane task of assuring reliability. The labs
desire the challenge of designing new nuclear weapons, simply for the scientific
and technical training experience the effort would bring. Hence, there is
tremendous pressure to create a new mission that justifies a new development
program.

But could the US
deploy a new low-yield nuclear earth-penetrating weapon without testing it? Under
continued political pressure to support the Test Ban and its related Stockpile
Stewardship Program, Los Alamos Associate Director Steve Younger has stated,
"one could design and deploy a new set of nuclear weapons that do not
require nuclear testing to be certified. However, ... such simple devices would
be based on a very limited nuclear test database."

On the other hand, it
seems unlikely that a warhead capable of performing such an extraordinary
mission as destroying a deeply buried and hardened bunker could be deployed
without full-scale testing. First, even if the missile casing were able to
withstand the high-velocity ground impact, the warhead "physics
package" and accompanying electronics must function under extreme
conditions. The primary device must detonate and produce a reliable yield
shortly after suffering an intense shock deceleration. Second, there must be
great confidence that the actual nuclear yield is not greater than expected.
Since the natural energy scale for a fission nuclear weapon is of order 10 KT,
much lower yield weapons must be sensitive to exacting design tolerances; the
final yield is determined by an exponentially growing number of
fission-produced neutrons, so the total number of neutron generations must be
finely-tuned. Given that these weapons may be used near population centers, it
thus seems highly unlikely that designers could certify a low-yield warhead
without actually testing it.

What would be the
consequence if the US decides to go ahead and test a new generation of nuclear
weapons? As House Democrats expressed in a letter to Rep. Ike Skelton of
Missouri, the ranking Democrat on the House Armed Services Committee,

"The resumption
of nuclear test explosions that will result from such a program involving
nuclear weapons would decrease rather than increase our national security and
undermine US and international non-proliferation efforts."

If the US abandons
the moratorium, Russia and China will almost certainly respond in kind 
destroying prospects for eventual passage of the CTBT.

Conclusion

Proponents of
building a new generation of small nuclear weapons have seldom been specific
about situations where nuclear devices would be able to perform a unique
mission. The one clear scenario is using these warheads as a substitute for
conventional weapons to attack deeply buried facilities. Based on the analysis
here, however, this mission does not appear possible without causing massive
radioactive contamination. No American president would elect to use nuclear
weapons in this situation  unless another country had already used nuclear
weapons against us.

The end of the Cold
War should allow us to place further limits on the development and use of
nuclear weapons. The danger of moving from a conventional to a nuclear war is
so enormous, that the US refrained from using nuclear weapons in Korea even
when US troops were in danger of being overwhelmed. Attempts to develop a new
generation of low-yield nuclear weapons would only make nuclear war more
likely, and they seem cynically designed to provide legitimacy to nuclear
testing - steps that would return us to the dangers of Cold War nuclear
competition, but with a larger number of nations participating.

Robert W. Nelson,
a theoretical physicist who works on technical arms control issues, is on the
research staff of Princeton University and a consultant to FAS.

3R = v2 / 2Y
= (v/vc) 2 where is the projectile
density, v is its velocity, Y is the yield strength of the
material, and the critical velocity vc = (2Y /)1/2

4For a penetrator which is much stronger than
the target, D/L (p / t)
ln(Yp / Yt), where L is the length of the
penetrator, is the material density, and Y is the material
strength to plastic yielding; the subscripts p and t stand for
the penetrator and target.